FIG. 1 shows a conventional arrangement of a cryostat including a cryogen vessel 12. A cooled superconducting magnet 10 is provided within cryogen vessel 12, itself retained within an outer vacuum chamber (OVC) 14. One or more thermal radiation shields 16 are provided in the vacuum space between the cryogen vessel 12 and the outer vacuum chamber 14. In some known arrangements, a refrigerator 17 is mounted in a refrigerator sock 15 located in a turret 18 provided for the purpose, towards the side of the cryostat. Alternatively, a refrigerator 17 may be located within access turret 19, which retains access neck (vent tube) 20 mounted at the top of the cryostat. The refrigerator 17 provides active refrigeration to cool cryogen gas within the cryogen vessel 12, in some arrangements by recondensing it into a liquid. The refrigerator 17 may also serve to cool the radiation shield 16. As illustrated in FIG. 1, the refrigerator 17 may be a two-stage refrigerator. A first cooling stage is thermally linked to the radiation shield 16, and provides cooling to a first temperature, typically in the region of 80-100K. A second cooling stage provides cooling of the cryogen gas to a much lower temperature, typically in the region of 4-10K.
A negative electrical connection to the magnet is usually provided to the magnet 10 through the body of the cryostat and a negative cable 21a. A positive electrical connection is usually provided by a positive cable 21 passing through the vent tube 20. In order to connect an external source of electricity to the positive cable 21, an electrical connection 22 must be provided through the wall of the turret outer assembly 32—and electrically insulated from the material of the cryogen vessel itself. Such electrical connections 22, commonly referred to as leadthroughs, are the subject of the present invention. The interior of the turret outer assembly 32 is exposed to the atmosphere of the cryogen vessel 12, typically helium in excess of atmospheric pressure.
The positive cable 21 must be electrically connected to an external source of electricity, yet the turret outer assembly must be sealed against cryogen leaks and air ingress. The leadthrough 22 is therefore required to provide electrical connection between the external source of electricity, and the positive cable 21 within the cryogen vessel. Such leadthrough must provide low resistance electrical continuity between the external source of electricity and the positive cable 21. It must provide a gas-tight seal to prevent cryogen gas in the cryogen vessel from escaping, and to prevent air ingress, through the seal. Helium is a commonly used cryogen, and the leadthrough must be made helium-tight if it is to be used in helium-cooled systems. The leadthrough must also provide electrical isolation between the material of the cryogen vessel and a conductive path between the positive cable and the external source of electricity. As mentioned above, it is common to use the body of the cryostat, including the material of the turret outer assembly 32, as the negative conductor to the magnet. The voltage applied to, or derived from, the magnet 10 will therefore appear across insulation provided as part of the leadthrough. In normal operation, such as introducing current into the magnet, or removing current from the magnet, the voltage across the magnet, and so across the insulation of the leadthrough, will be no more than about 20V. It is relatively simple to provide electrical isolation effective at such voltages. However, in the case of magnet quenches, where a superconductive magnet suddenly becomes resistive, large voltages may be developed across the coils of the magnet. In such circumstances, voltages reaching about 5 kV may appear across the insulation of the leadthrough. In any such leadthrough it is therefore necessary to provide electrical isolation sufficient to withstand an applied voltage of several kilovolts. Furthermore, during filling of the cryogen vessel with liquid cryogen, or in the case of liquid or boiling cryogen being expelled from the cryostat during a quench event, parts of the leadthrough exposed to the interior of the cryogen vessel may be cooled to a temperature of about 4.2K, the boiling point of helium. At the same time, parts of the leadthrough exposed to ambient temperature may be at 300K or more. Any leadthrough must therefore be able to withstand temperature differences of over 300K without deterioration.
FIGS. 2A and 2B show a known leadthrough as currently used to carry electricity into a cryogen vessel, in schematic cross-section, and in schematic perspective cross-section. A leadthrough conductor 30 is electrically isolated from a wall of the turret outer assembly 32 by a ceramic seal 34. An outer stainless steel fitting 36 seals against the leadthrough conductor 30 and the ceramic seal 34, retaining the ceramic seal in position, spaced concentrically away from the conductor 30. An inner stainless steel seal 38 seals against the ceramic seal 34 and extends radially away from the conductor 30 to provide a rim 40, radially spaced away from the conductor 30. In use, the leadthrough is welded by rim 40 to the turret outer housing 32. By having rim 40 spaced away from conductor 30, the risk of short circuiting the conductor to the rim during welding is reduced. The thermal distance between the weld location at rim 40 and the ceramic seal 34 needs to be sufficient to avoid thermal damage to the ceramic seal. Current lead 21, shown as a flexible metal laminate in the drawings, may be attached to the inner end of conductor 30 by any suitable fixing, such as a simple through-hole 41 and nut 58 on a threaded end 56 of conductor 30 as shown.
Generally, such arrangement has been found to provide satisfactory electrical performance and satisfactory sealing. On the other hand, such ceramic seals 34 have been known to fracture due to mechanical or thermal stress. Fracture of the ceramic seal may lead to contamination of the cryogen vessel with ceramic particles, a leak of cryogen gas to atmosphere, or ingress of air into the cryogen vessel. In a recent development, leadthroughs such as shown in FIGS. 2A, 2B are provided with an external support structure, which acts to mitigate some of the effects of fracture of the ceramic seal, but does not address the inherent mechanical weakness of the existing leadthrough.
Ceramic seals such as currently used in leadthroughs such as shown in FIGS. 2A and 2B cost about GB£200 (about US$400).
If a ceramic seal 34 such as shown in FIGS. 2A, 2B should break, it is necessary to cut the weld between rim 40 and wall 32, to clean out any contamination of the cryogen vessel and replace the leadthrough, including welding the rim 40 of the new leadthrough to the wall 32. Such operation has been known to cost in the region of £2500 (US $5000). If failure of the ceramic seal at a customer site causes return of the cooled equipment and cryostat, much higher costs may be anticipated.
It is an object of the present invention to provide a leadthrough suitable for providing electrical connection between a current lead within a cryogen vessel and an external source of electricity, which is gas-tight, which are not susceptible to fracture due to mechanical or thermal stress, which provides a significant cost saving over the currently available leadthroughs which use ceramic seals, and preferably which is simple to install and replace.